I am stuck understanding this question using chain Rule

Question

Suppose thatf(x)=3x(5−4x)^3.

Find an equation for the tangent line to the graph of f at x=1.

y=

Answer 1

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Answer 2

f(1) = 3(5 - 4)³ = 3; f'(x) = 3(5 - 4x)³ - 12x(5 - 4x)²; f'(1) = 3 - 12 = - 9;

Tangent line: y = 3 - 9(x - 1); y = - 9x + 12